1. Technical Field
This invention relates to methods and apparatus for improving the signal-to-noise ratio of ultrasonic sensors utilized for sensing fluid flow within a pipe in general, and to methods and apparatus for lensing an ultrasonic beam or signal to optimize the signal properties of the transmitted ultrasonic beam passing through a fluid flow having entrained particles or bubbles.
2. Background Information
Multiphase fluid flows pose certain issues for flow meters utilizing ultrasonic sensors. In addition to other factors, such as absorption and reflections, particles and/or bubbles entrained within a fluid flow will attenuate an ultrasonic signal or beam propagating through the fluid. The attenuation of the signal may result in a low signal-to-noise ratio, which in turn can make it more difficult to accurately detect and measure the transmitted ultrasonic signal.
For those ultrasonic flow meters that utilize transmitter/receiver sensor units disposed outside the flow pipe the signal-to-noise ratio of the device can also be negatively affected by undesirable “secondary” signals that are portions of the original signal that have been refracted or reflected along a different path through the pipe instead of through the fluid flow. These secondary signals often possess sufficient strength to reach the receiver transducer of the sensor unit, and may interfere with the desired signal passing through the fluid flow. Examples of undesirable secondary signals include “ring-around” signals that travel through the pipe wall.
FIG. 12 diagrammatically illustrates an ultrasonic sensor arrangement where ring-around signals and through-transmitted signals are generated in a sensor unit having an ultrasonic transmitting sensor 40 and the ultrasonic receiving sensor 42 of a sensing unit attached to the exterior surface of a pipe 14. The sensors 40, 42 are arranged such that the generated ultrasonic signal 15 is injected normal to the pipe 14 (and the flow path through the pipe), with the intent to have the injected signal travel through the center of the fluid within the pipe 14. As the ultrasonic signal 15 bisects the pipe 14, bubbles 17 and other matter within the pipe 14 will scatter and attenuate the signal before it fully traverses the pipe 14 and is detected by the receiving transducer 42. At the same time, ring-around signals 19, created through reflection and diffraction between the ultrasonic transmitter and the wall, travel within the pipe wall and are sensed by the receiver 42 along with any desired signals. The noise created by the secondary signals (e.g., ring-around signals) coupled with the significant attenuation of the through-transmitted signal, creates an undesirable signal-to-noise ratio that makes it difficult to accurately detect and measure the desired through-transmitted ultrasonic signal.
What is needed is a method and apparatus operable to optimize the signal properties of an ultrasonic signal to improve the detection of the ultrasonic signal under various flow conditions related to particles and bubbles entrained within the fluid flow, and one which thereby diminishes the effects of undesirable noise resulting from secondary signals.